1. Field of the Invention
The present invention relates to a method for the synthesis of very fluid overbased additives having a high basicity.
Overbased additives are alkaline-earth metal salts of organic acids, overbased through carbonation with carbon dioxide. The term "overbased" is used to designate the excess of alkaline-earth metal with respect to the stoechiometric quantity necessary to neutralize the organic acid used.
2. Description of the Prior Art
These overbased additives have been used in lubricating oils for many years. They have the property of neutralizing the acidity developed in motors by the combustion of sulfur-based organic derivatives contained in the fuels as well as by the oxidation of the components of the oils that occurs during running of the motors employed in land vehicles or marine craft.
The use of fuels containing increasingly high amounts of sulfur, in particular fuel for heavy fuel motors, associated to increasingly stringent operating conditions, renders indispensable the use of lubricating compositions capable of neutralizing the large quantities of acids formed during combustion.
The overbased additives are also applied as anticorrosive agents in burner boilers using fuel since they preferentially complex the vanadium compounds and oxides present in the fuels. The compound that is normally deposited on the walls of furnaces during combustion is at the origin of the oxidation and the corrosion of the furnaces. The alkaline-earth metals, and especially magnesium, form with vanadium oxides high melting point eutectics, which do not stick to the wall of the furnace pipes.
The basicity of these overbased products is characterized by their alkaline value AV which is expressed in milligrams of KOH per gram of overbased additive titrated by a strong acid according to ASTM D 2896 standard.
The overbased additives must have high alkaline values, generally higher than or equal to 250. Thereafter, the viscosity of these products must be sufficiently low to facilitate their handling and utilization. Furthermore, they must be translucid without any trace of mineral particles in suspension. The solids in suspension will hamper the effect desired by provoking an abrasion of the motors and burners. Furthermore, they must conserve their limpid aspect and retain a homogeneous consistency with time.
Numerous processes for the preparation of overbased additives already exist.
European patent application No. 005337 describes a process that consists in carbonating an alkylarylsulfonate and magnesium oxide mixture in a diluent oil and in xylene, in the presence of methanol, water and ammonia.
According to British patent application No. 2 055 885 methanol is replaced by ethoxyethanol, thereby allowing overbased products having reduced viscosity to be obtained.
British patent applications No. 2 114 993 and 2 037 310, and European patent application No. 013 807 as well as French patent No. 2 528 224 claim replacing methanol by respectively dioxolan, a methanol-carbon dioxide mixture, a methanol-diacetone mixture or diglycol.
Furthermore, U.S. Pat. No. 4,347,147 claims the use of ethylenediamine as a substituent of ammonia.
U.S. Pat. No. 3,929,217 proposes a method of carbonation without the addition of water.
All these processes are difficult to reproduce commercially and often lead to doughy or solid products. In other cases, low alkaline values (AV) are obtained that do not allow the obtention of the desired performances in the motors or in the furnaces. The processes described for an alkaline-earth metal cannot be transported to other metals due to the difference in reactivity of these other metals.
A simple and commercially reproductible process has now been developed that allows the obtention of overbased additives having very high alkaline values and low viscosity, this process being industrially feasible, whatever the type of alkaline or alkaline-earth metal used.